Wind turbines typically include a rotor with large blades driven by the wind. The blades convert the kinetic energy of the wind into rotational mechanical energy. The mechanical energy is typically transferred via drive train to a generator, which then converts the energy into electrical power.
Most modern wind turbines control power output by pitching the blades relative to the wind. Thus, each blade is mounted to a hub by a blade bearing that allows relative movement between the blade and the hub. The blades are rotated about their longitudinal axis by a pitch system that includes one or more electrical drives (e.g., electrical motors) or hydraulic drives (e.g., hydraulic actuators). Although a single drive may be used to collectively pitch the blades, the pitch systems in most modern wind turbines include drives for pitching each blade individually.
There are various ways of implementing a pitch system. Most designs, however, mount the blade to an inner ring of the blade bearing and the hub to an outer ring of the blade bearing. Such an arrangement makes it easier connect the pitch drive, which is typically mounted to the hub, to the blade via rods, gears, or other mechanical links. Additionally, the elements of the pitch system in such an arrangement can in large part be contained within the interior of the hub (and possibly blade), which is generally desirable. And finally, such an arrangement helps ensure that the hub remains large enough to have the necessary structural rigidity to withstand loads and/or have the necessary interior space to accommodate service personnel.
Over time there has been a significant increase in the overall size of wind turbines because of the desire to capture more of the wind's available energy. As the hub—typically a casted component—has become larger, material costs present as much concern (or more) as the hub being a sufficient size for structural integrity. Mounting the inner rings of blade bearings to the hub rather than the blade represents an opportunity to decrease the size of the mounting areas and overall body of the hub, thereby saving material costs.
Although several designs have been proposed for mounting the hub to the inner ring of a blade bearing and the blade to the outer ring, there remains room for improvement. For example, connecting the hub-mounted pitch drive to the outer bearing ring/blade presents a challenge. Attempts to address this challenge have focused on mounting elements of the pitch system on the exterior of the hub. This is generally not considered practical for hydraulic pitch systems because of the stroke length of the actuator and mechanical links that would be needed to provide the required range of pitching. Only a compact electrical motor acting on a toothed ring connected to the outer bearing ring or blade is a viable option.
Indeed, for electrical pitch systems, such exterior placement is generally considered the only viable option for reasons relating to service and maintenance. The toothed ring is subject to wear and typically requires replacement during the lift time of the wind turbine. If located in the interior of the blade (e.g., mounted to an inner wall of the blade or the top of the outer bearing ring), the blade must typically be removed to replace the toothed ring. Thus, the complexity, costs, and downtime associated with the replacement of the toothed ring is significantly greater compared arrangements where the toothed ring is exterior-mounted such that blade removal is not required.